As the magnetic storage of information becomes more sophisticated, greater amounts of data are packed into smaller volumes of space. In storing large amounts of data on a magnetic tape, multiple tracks of data are stored paralleling the length of the tape. The number of tracks that can be stored on a particular width of tape depends on the sensitivity of the technology used. Currently it is possible to have several hundred tracks on a ½ inch wide magnetic tape. The accuracy of how well the read/write head can be properly positioned depends on several factors.
A major hurdle in recording and reading data from the hundreds of tracks on the magnetic tape is the lateral movement of the tape media as it traverses the read/write head. This is overcome by manufacturing the tape with recorded tracks of servo information at various lateral locations across the tape. These servo tracks provide information that allows the servo mechanisms in a tape transport to correctly position the magnetic tape head with respect to the data tracks.
One method of coding information in the patterns written in the servo tracks is timing based servo (TBS) coding which is insensitive to reading speed and provides a method of generating a tracking error signal to accurately position a read/write head on tape media. The TBS method provides that each servo track is written with a repeating cyclic sequence of patterned lines whose separation from each other varies in a consistent manner across the width of the servo track at the time of the magnetic tape manufacture. The periodic gaps are written at the time the cartridge is manufactured using a formatter head which writes a series of servo bands down the length of tape.
Servo bands written on the tape are monitored and followed to ensure that the drive's read/write heads are correctly placed on the tape. Repeatable vertical data track placement is essential to avoid errors that may occur when reading and writing data. A misplaced read/write head could read data from an adjacent track during a read operation or when writing data it is possible to overwrite existing data on an adjacent track. Correct data track placement assumes that the servo edges are a predetermined distance apart. The magnetic tapes are servo formatted to achieve such predetermined spacing. The servo readers on the magnetic tape head are also a predetermined distance apart. Variations in the servo bands may result in a data track placement error.
Tape drives often use the TBS format which determines the timing from edge to edge between two servo patterns, specifically an A and a B pattern. It is this edge to edge timing in conjunction with drive microcode that determines the position of the magnetic tape head on the tape. The A and B patterns are made up of individual stripes that include leading and trailing edges and are written by two separate servo format gaps. It is desirable to write the stripes of the same width, but due to head, coil, and electronic tolerances, the widths of these stripes can differ. This difference can introduce errors which will induce a positional offset of the magnetic tape head relative to the tape which can begin to encroach on adjacent tracks. If the stripe widths are significantly different, the read/write head can be positioned such that it can overwrite a portion of the adjacent track which can lead to data detection errors, data loss or the like.
The gain of the servo algorithms is such that even small difference in the stripe widths can cause a meaningful amount of track misregistration (TMR) in the tape tracks. As the track densities become higher, the track misregistration budgets become tighter and the TMR component becomes a larger percentage of the TMR budget.
It would be desirable to have a method and system for servo stripe width detection and compensation that would overcome the above disadvantages.